This invention relates to an exhaust gas recirculation system (EGR system) for reducing NO.sub.x (Nitrogen Oxides) emmissions, especially to the improvement of the back pressure controlling type EGR system.
The known back pressure controlling type EGR system comprises:
A diaphragm type EGR valve mounted on a EGR pipe which connects the exhaust pipe with the intake pipe of the engine; PA1 A pressure control chamber formed at the inlet of said EGR valve; PA1 An orifice mounted on a communicating pipe which connects the diaphragm chamber of said EGR valve with the EGR port arranged near the throttle valve of the carburetor; PA1 A relief pipe diverging from said communicating pipe between said orifice and said EGR valve, and; PA1 A vacuum control valve (VCV) mounted at the opening end of said relief pipe. PA1 A first diaphragm; PA1 A second diaphragm; PA1 A first chamber which is formed outside said first diaphragm and communicating with the atmosphere; PA1 A second chamber which is formed between said first and second diaphragms and communicating with the pressure control chamber of the EGR valve, and; PA1 A third chamber which is formed outside said second diaphragm and communicating with the intake pipe of the engine at the downstream of the throttle valve of the carburetor.
In this known EGR system, said vacuum control valve (VCV) comprises a diaphragm and two chambers defined by the diaphragm and the valve body, one of the chambers communicating with the atmosphere and the other communicating with said pressure control chamber of the EGR valve.
In the above EGR system, the amount of the EGR is controlled by the EGR valve actuated by said VCV which operates so that the pressure in said pressure control chamber keeps constant. Such operation maintains the EGR ratio (the ratio of the amount of the recirculated exhaust gas to that of all the exhaust gas) constant no matter what the engine condition is. Accordingly, in the low-loaded condition of the engine the exhaust gas is recirculated using the same ratio as in the high-loaded condition of the engine in which a large amount of the exhaust gas is recirculated in order to clean the discharged exhaust gas. Consequently, in the low-loaded condition of the engine, an excessive amount of exhaust gas is recirculated with the result of degrading driveability of the vehicle and lowering the output force of the engine.